Method and article for facilitating the manufacture of a bondable metallic surface

ABSTRACT

A pad eliminatable by heat prior to casting has a patterned surface and a non-porous backing. The pad is sized and shaped into at least part of the appliance and the patterned surface is conformed to the desired bonding surface contour. The patterned surface yields irregularities in the cast bonding surface which increases bondability.

This is a division of application Ser. No. 438,558, filed Nov. 2, 1982,and now U.S. Pat. No. 4,479,527.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to casting and more particularlyto an article having a pattern which is to be incorporated into thebonding surface of an appliance during its manufacture by investmentcasting and its method of incorporation.

2. Description of the Prior Art

The present invention is applicable to the casting of metallicappliances or devices requiring a surface which facilitates bonding byan adhesive, and is particularly suitable to the manufacture of dentalappliances.

Bonding of intraoral dental appliances to the surfaces of teeth isfrequently necessary during orthodontic treatment. Proper bonding withan adhesive necessitates the preparation of both the bonding surface ofthe appliance and the tooth surface. Conventionally, the tooth enamel isacid etched to enhance the bonding of the adhesive to the tooth. Thebonding surface of an orthodontic appliance may be prepared by attachinga porous mesh bonding pad to the appliance by brazing, welding orsintering.

The surface of a cast dental appliance may be electrolytically etched.This approach is used in what has become known as the Maryland Bridgetechnique, popularized by Drs. Thompson and Livaditis at the Universityof Maryland. The Maryland Bridge typically consists of a central ponticwith wing-like retainers at each end for attachment to abutment teeth.The bonding surface of the Maryland Bridge retainers are acid etched tocreate an irregular surface which increases its bondability to thesurface of a tooth. In contrast with the earlier and traditional crownand bridge techniques the two abutment teeth, one on each of theopposite sides of the pontic, are not ground down, and the need for apartial denture is eliminated.

Disadvantages of the Maryland Bridge technique are that only the lesscorrosion resistant base metal alloys can be satisfactorily etched, andthe etching process is itself difficult to control as the extent andevenness of the etch are dependent on the current, voltage, surface areaand acid strength.

Problems that dental laboratories are faced with when practicing thistechnique involve the use of acids. Some dental laboratories areunaccustomed to using corrosive acids in their routine operation andsuch fluids may cause accidents. In addition, acid fumes requiresuitable exhaust ventilation, e.g. a chemical fume hood, which may belacking in many smaller laboratories. A further drawback is that theequipment for etching is expensive, as it includes at a minimum anelectroplating device with a stirrer and timer.

It is a primary object of the present invention to simplify themanufacture of an appliance having a bonding surface withirregularities, without using acid etching or the addition of a bondingpad to the cast article.

It is another object of the present invention to cast an appliance as asingle element with a plurality of irregularities formed into the toothabutting surface of the appliance.

It is also an object of the present invention to provide a preformed padwhich is beneficially incorporated into a mold to form a patternedbonding surface.

It is a further object of the present invention to provide a devicecapable of producing a patterned bonding surface in corrosion resistantalloys, as well as etchable metallic alloys.

It is an additional object to provide a patterned bonding surface to anorthodontic appliance which achieves greater bonding strength with toothenamel than devices presently available.

It is also an object of the present invention to simplify and reduce thetime necessary for the fabrication of a dental bridge.

SUMMARY OF THE INVENTION

These and other objectives are accomplished by the present inventionwhich includes a novel article structured for use in the lost waxinvestment casting process and the method of incorporating the article.The article is a pad which is eliminatable from the investment castingmold at temperatures typically employed during the lost wax process.These temperatures typically range from 1400°-2000° F. The pad includesa surface portion that is predeterminedly patterned so as to produceasperities in the casting surface derived from the patterned surfaceportion. The patterned surface portion has a backing portion affixedthereto which is non-porous.

The patterned surface portion is flexible so as to be conformable to apredetermined surface, but with sufficient rigidity to do so withoutcreasing. When manufacturing dental appliances, the predeterminedsurface is typically taken off of a dental model and corresponds to thelabial or lingual surface of the tooth to which the appliance is to bebonded. After conformation to the dental model, the patterned surface isfixed in shape so that upon removal from the predetermined surface, thepatterned surface portion maintains the conformed surface contour. Thefixation is provided at least in part by the non-porous backing portion.If desired, additional material, such as wax, may be added to thebacking portion to provide additional rigidity, and/or the new materialmay be added for sculpturing to the shape of the dental appliance.

A method utilizing the above described pad for incorporation in thecasting of an appliance so as to produce an irregular, asperity-riddensurface which facilitates adhesive bonding includes the following steps:sizing at least one pad so that the patterned surface portionapproximates at least a portion of an appliance to be cast; attachingthe patterned surface with suitable adhesive to a predetermined surfaceto be replicated; fixedly conforming the surface portion of the pad tosaid predetermined surface; investment casting said pad; and eliminatingsaid pad from the casting mold with the heat employed during the lostwax investment process.

A particularly suitable use of the pad is in the fabrication of thewings of the Maryland Bridge.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, characteristics and advantages of thepresent invention will be more clearly understood from the followingdetailed description when read in conjunction with the accompanyingdrawings in which:

FIG. 1 is a front elevational view of a two-layered mesh pad inaccordance with the present invention;

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a front elevational view showing an alternative shape for amesh pad, in accordance with the present invention;

FIG. 4 is a front elevational view showing a three-layered mesh pad withwider mesh openings than FIGS. 1-3 and an alternative shape;

FIG. 5 is a cross sectional view taken along line 5--5 of FIG. 4;

FIG. 6 is a perspective view from the lingual side of a portion of adental model with the periphery of the abutment teeth on opposite sidesof a missing tooth outlined on the model by a marker;

FIG. 7 is a perspective view from the lingual side showing the outlinedshape of a tooth from a dental model being traced with a utensil on thepad backing;

FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 7;

FIG. 9 is a perspective view from the lingual side showing the abutmentteeth with the pads properly sized and affixed;

FIG. 10 is a cross-sectional view taken along line 10--10 of FIG. 9;

FIG. 11 is a perspective view from the lingual side showing each of thetwo abutment teeth fitted with a pad and affixed to a wax pontic;

FIG. 12 is a cross-sectional view taken along line 12--12 of FIG. 11;

FIG. 13 is a perspective view of a disposable bridge casting supportsprue with the pontic and retainer wings attached thereto;

FIG. 14 is a perspective view from the labial side showing the castpontic and retainer wings positioned for insertion according to thearrows;

FIG. 15 is a perspective view from the labial side of the pontic andretainer wings in place, as indicated by the phantom lines;

FIG. 16 is a cross-sectional view taken along line 16--16 of FIG. 15;

FIG. 17 is a perspective view from the lingual side showing the priorart Maryland Bridge;

FIG. 18 is a photomicrograph showing the tooth abutting surface of theetched wing of a Maryland Bridge at 9× magnification; and

FIG. 19 is a photomicrograph showing the bonding surface of an articlemanufactured with a pad in accordance with the present invention at 9×magnification;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the Figs. wherein a pad in accordance with the presentinvention is generally indicated by the numeral 20. The pad includes asurface portion 22 at least partly patterned 24 and a non-porous backingportion 26. The entire pad 20 is composed of material or materials whichare eliminatable by the temperatures conventionally used in the lost waxtechnique of investment casting, typically in the range of 1400°-2000°F.

In FIGS. 1-4 the patterned surface portion 24 is a mesh or screenappropriate for the manufacture of a bonding surface for dentalappliances. Preferably, the mesh is a Poly-Sieve woven polymer made of apolyethylene fiber. Other appropriate mesh materials are nylon andpolyester. A wide range of mesh openings are suitable, including therange of 100 to 1,300 microns, and in fact mesh opening sizes of 202microns and 432 microns produced a cast bonding surface havingacceptable bond strengths. It has been found, however, that the 202micron mesh size conforms more readily to the surface of a tooth modelto be replicated by casting. For dental applications, the thickness ofsuch preformed patterns typically range from 0.2 to 1.0 mm, with apreference at about 0.5 mm. The backing portion is preferably anon-porous pressure sensitive wax. A gauge of 26 has been found to beparticularly suitable. The backing portion is affixed to the patternedsurface portion 24 by an appropriate adhesive, e.g. 3M 4693 plasticadhesive.

Numerous pattern materials are appropriate for the surface portion 22.In fact, the predeterminedly patterned surface portion 24 need notcontain openings. The pattern 24 must, however, contain an adequatenumber of indentations and/or protrusions to provide a roughened bondingsurface in the cast article derived therefrom.

In addition, the pattern must be suitably flexible to conform to theshape of the surface requiring replication without creasing. The padsare preferably preformed into shapes suitable for dental applications.An alternative shape for a preformed pad is shown in FIG. 3.

FIGS. 4 and 5 show a pad 20 having a surface portion 22 with apredetermined surface pattern 24' having wider mesh openings than thepad of FIGS. 1-3. The backing portion 26 affixed thereto is providedwith a barrier layer 28 composed of a material which prevents thepressure sensitive wax portion 30 from penetrating the mesh, andotherwise diminishing the bonding strength of the cast bonding surface.Cellophane and polypropylene are very effective materials for thebarrier layer 28. Pressure is applied to the wax portion 30 when the padpatterned surface portion 24 is conformed or shaped to the surfacerequiring replication.

A particularly preferred embodiment of this three type layer of padincludes a 202 micron woven Poly-Sieve screen as the surface patternedportion 24, and a barrier layer 28 of a clear polypropylene plasticsheet 0.0022 mm thick attached to the screen by an adhesive, and anon-porous 26 gauge pressure sensitive wax affixed to the barrier layer28, also with an adhesive. An appropriate adhesive for both interfacesis 3M plastic adhesive 4693.

A preferred use of the pads involves the fabrication of a bridge. FIG. 6is a schematic partial view of a stone dental model, generally indicatedby the numeral 32, of a patient's mouth, as seen from the lingual side.A space 34 exists where a tooth is missing. The periphery of the twoabutment teeth 36 are outlined 37 on the dental model 32 and a preformedpad 20, in this case the two-layered embodiment depicted in FIG. 3, isselected from an assortment of pads for each of the abutment teeth 36.The pads 20 should approximate, but yet be larger than, the shape of thedesired retainers or wings 38, one of which will be positioned on eachside of a fabricated pontic 40 (FIG. 11). The patterned surface portionor mesh 24' will correspond to the bonding surface of each retainer 38.As shown here, the bonding surface of each retainer wing 38 willcorrespond in surface area and shape of the lingual side of eachabutment tooth 36.

As shown in FIGS. 7 and 8, the pads 20 are placed against the abutmentteeth 36 of the model 32 with the patterned mesh portion 24' against thelingual surface. The outline 37 of the periphery marked on the dentalmodel 32 is traced by an appropriate tool 42 onto the non-porous backingportion or wax side 26 of the pad 20. The pads 20 are removed and cutalong the traced line with an appropriately curved scissor, such as theUnitek Curved Scissor Model 801002, so as to yield a pad correspondingto the exact size of the lingual side of each of the abutment teeth 36.

An appropriate temporary adhesive 43 is applied to each of the abutmentteeth 36 so as to cover the entire lingual side. Each of the pads 20 arethen properly positioned on the abutment tooth 36 with the patternedmesh surface portion 24' against the adhesive 43, as shown in FIGS. 9and 10. Thus, the mesh or patterned surface portion 24 of each pad 20will contact the adhesive 43 and the backing portion 26 will extendtoward the inner portion of the dental model 32.

The pads 20 are conformed to the predetermined lingual surface byapplying pressure to the backing portion 26 of the pads 20 with anappropriate utensil.

Referring now to FIGS. 11-16, a preformed wax pontic 40, built in amanner well-known in the art, is positioned in the space 34. Anappropriate non-porous wax, which may be the same wax as used in thebacking portion 26, is then applied at 42 to attach the pontic 40 to thebacking portion 26 of the pads 20 affixed to each of the model abutmentteeth 36. More non-porous wax may be added to the backing portion 26 ofthe pads 20 and pontic 40 to provide additional rigidity and/or to besculptured into the desired shape of the retainers 38. At this point,the three part bridge, composed of two wings or retainers 38 sandwichinga pontic 40 and generally designated by the numeral 44, appears as shownin FIGS. 11 and 12, and is referred to as a wax-up.

The stone dental model 32 is soaked in water and the three part wax-up44 of the bridge is released. The temporary adhesive 43 is removed fromthe patterned surface portion or mesh 34 by brushing the mesh 24' undercold running water. The wax-up 44 is then prepared for investmentcasting using the lost wax process which is well-known in the art.However, prior to complete investment, the investment material, e.g.Vestra™, is gently worked into the mesh openings of the patternedsurface portion 24' so as to insure duplication of the mesh pattern inthe cast bonding surface on the bonding surface of the retainers 38.

FIG. 13 shows an investment casting 45 of the wax-up 44 mounted on adisposable bridge casting support 47 of a type known in the art, withthe appropriate sprues 39 and vents 41 still attached.

The wax up of the investment casting 45 is removed preferably with heat.This is typically done at the aforementioned temperatures of 1400°-2000°F., appropriate to the lost wax process. A solid metal casting 45' (FIG.14) of the dental bridge is then prepared from the investment castingwhich is composed of the pontic 40', and metal retainer wings 38'. Thepontic portion 40' is coated with porcelain 48 by known methods so thatthe bridge appears as shown in FIG. 14, and indicated by the generalnumeral designation 47. The finished bridge is positioned for insertionin the patient's jaw between the patient's abutment teeth 36'.

As shown in FIGS. 15 and 16, the bridge 47 is not visible from thelabial side as the metallic mesh patterned bonding surface of each metalretainer 38', derived from the patterned surface portion 24 of the pad20, abuts the lingual surface of an abutment tooth 36'.

A permanent adhesive 51 is placed on the lingual side of the abutmentteeth, and the wings 38' are affixed thereto.

FIG. 17 shows the prior art Maryland Bridge 50 with acid etched retainerwings 52 as seen from the lingual side positioned for insertion into thepatient's jaw 49.

FIGS. 18 and 19 show the contrast between an etched wing 52 (FIG. 11) ofthe prior art Maryland Bridge, and the cast mesh patterned bondingsurface of the wing 38' produced with a pad 20 of the present invention.Each is shown at a 9× magnification. Clearly, the surface irregularitiesof the cast mesh pattern retainer wings 38', achieved with the aid ofthe pads 20 of the present invention, are of greater depth and frequencythan those of the etched wings 52'. Test results indicate that superiorbonding is achieved with the pad of the present invention.

A peel/shear bond strength test was used to determine the bond strengthof dental appliances prepared using the present invention and method ascompared to the bonding techniques presently available. According to thetesting procedure, bovine teeth are mounted in acrylic so as to exposethe labial surface on which test samples are bonded, using etching andbonding techniques. A no-mix adhesive known as Unite, a trademark ofUnitek Corp., and a conventional adhesive known as Concise, a trademarkof 3M Company, were used. The acrylic mounts are placed vertically in anInstron Universal Testing Machine. The test samples were peeled/shearedoff the bovine tooth through a wire looped around the "bracket" side ofthe appliance. The following three types of orthodontic brackets weretested: (1) an acid etched base; (2) cast mesh base; and (3) sinteredwire base. In addition, etched and cast mesh wing portions of a bridgecustomized to bovine teeth were evaluated. As shown in Table I below,the predeterminedly patterned or cast mesh porous surface produced inaccordance with the present invention exhibited the highest bondstrength in both types of test samples, and tooth fracture was oftenencountered.

                  TABLE I                                                         ______________________________________                                        Type of Bonding                                                                              Average Bond Strength (lbs.)                                   ______________________________________                                        Acid etched bracket base                                                                     26                                                             Cast mesh bracket base                                                                       62                                                             Sintered wire mesh base                                                                      41                                                             Acid etched wings                                                                            84                                                             Cast mesh wings                                                                              exceeds 143 or fracture                                                       strength of the tooth                                          ______________________________________                                    

It should be noted that the cast mesh retainers of wings are larger incontact area than the rest of the bonding bases. Generally, twelve testswere conducted on each type for averaging. In the twelve tests conductedon the cast mesh retainers, test fixture wire failure, tooth failure andtooth holder failure occurred repeatedly and, consequently, only a lowerlimit for the bond strength can be established.

While the invention has been described above with respect to specificembodiments, it should be clear that these embodiments are given by wayof example and shall not be deemed as limiting the scope of theinvention, except in accordance with the claims hereof.

The invention claimed is:
 1. An article for use in the lost waxinvestment casting process, comprising;a pad having a surface portion atleast partly predeterminedly patterned for producing asperities andincreasing bondability in a cast surface derived therefrom, and anon-porous portion backing affixed to said patterned portion, said padbeing eliminatable at temperatures employed during the lost waxinvestment casting process, said patterned portion being fixedlyconformable to, and releasably attachable, by a bonding agent to apredetermined surface.
 2. The article of claim 1 wherein said patternedsurface portion is composed of different material than said backingportion.
 3. The article of claim 1 wherein said backing portion includesa pressure sensitive wax.
 4. The article of claim 1 wherein said surfaceportion has a mesh pattern.
 5. The article of claim 4 wherein said meshis a woven polymer.
 6. The article of claim 1 wherein said backingportion includes a barrier layer abutting said patterned surfaceportion.
 7. The article of claim 1 wherein said predetermined surface isa model of a tooth surface.
 8. The article of claim 1 wherein said padshave predetermined shapes utilizable for dental applications.